drm/i915/hdmi: Add missing intel_pfit_mode_valid() for 4:2:0 also modes

drm/i915/hdmi: Add missing intel_pfit_mode_valid() for 4:2:0 also modes

intel_hdmi_mode_valid() is calling intel_pfit_mode_valid() only
on the first attempt (4:2:0 for "4:2:0 only" modes, 4:4:4 for
everything else). Add the call also for the "4:2:0 also" modes case
so that we actually know the pipe scaler can actually produce the
4:2:0 output.

Cc: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260409101539.22032-2-ville.syrjala@linux.intel.com
Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>